A study using unlabelled 10 micron m black microspheres will be made in monkeys to determine the effect of increments in eye pressure on the blood flow in the retina and the different parts of the optic nerve. Percentage changes in regional blood flow are determined from differences in the amount of spheres recovered in sections. The effect of sympathetic stimulation on the blood flow in the brain and the different parts of the eye will be determined in monkeys under conditions of acute arterial hypertension. Blood flow is determined with radioactive microspheres and breakdown of barriers is determined with Evan's blue or fluorescein. Experiments will be made at normal oxygen tensions and in hypoxia. An analysis will be made of the mechanism that causes marked vasoconstriction in the anterior uvea and dilatation of the blood vessels in the choroid when the oculomotor nerve is stimulated intracranially in rabbits. The nerve is stimulated stereotactically in sympathectomized animals. It will be determined also whether light activates the mechanism. A study will be made on the effects of oculomotor nerve stimulation on the rate of aqueous formation in monkeys and rabbits. Aqueous formation is determined with an isotope dilution method. The effect of indomethacin on the response will be studied in monkeys. The permeability of the choroidal capillaries and the pigment epithelium and the retinal capillaries is studied in cats. Radioactive substances are injected intraarterially, and delay of the substances and extraction in the tissues is determined from analysis of choroidal and retinal venous blood.